You can still play the main campaign if your DLC is unchecked in the "Downloaded Content" tab.
BioWare staff has come out to say that their servers got borked, and that they're working on it. They created four threads called "'Unauthorized DLC' error message", one in each of the tech support forums (PC, Mac, PS3, 360). Here's the DA:O PC Support forum.
Unfortunately they didn't lock the threads, and now they're all huge. It doesn't seem like BioWare staff has posted anything other that the initial announcement. Navigate through the threads at your risk.
Servers went down on Friday April 8th, BioWare's posts came round noon of Monday April 11th.